From Motor Control to Agricultural Machinery Retrofits: My Field Observations on HASL PCB Applications

I recall visiting a factory where I encountered a batch of boards undergoing rework due to issues stemming from the HASL stage. The workers, out of habit, had applied the temperature profile designed for leaded solder to process lead-free solder. Consequently, the edges of the solder pads resembled burnt cookies—brittle and prone to crumbling at the slightest touch. The root of the problem, it turned out, lay not in the sophistication of the equipment, but rather in the operators’ understanding of material properties—an understanding that remained stuck in the past. Nowadays, many small factories still use lead-based solder to save trouble; after all, it can cut costs by a third. However, consider this: if the final product is exported to Europe and customers detect excessive heavy metals, the financial losses from forfeiting the entire order wouldn’t even be enough to cover the damages—let alone the irreparable damage to the brand’s reputation.

I actually believe that the slow pace of the transition to lead-free manufacturing isn’t due to high technical barriers, but rather the lack of coordination across the upstream and downstream segments of the supply chain. For instance, some board suppliers provide copper substrates with surface roughness levels that simply do not meet the requirements for lead-free hot air solder leveling (HASL). When these boards are forced onto the production line, the hot-air leveling process inevitably results in several spots where the solder fails to adhere properly.

A Shenzhen-based company we collaborated with last year learned this lesson the hard way. They assumed that simply switching their solder alloy to a lead-free alternative was enough to meet environmental compliance standards. However, during mass production, they discovered that their soldering yield rate had plummeted to below 70%. Upon disassembly and inspection, they found that bubbles had formed between the solder-leveled layer and the solder mask; they later traced the issue back to an incompatibility between their flux formulation and the new surface plating.

In reality, what truly requires adjustment is the entire production rhythm. Lead-free processes impose almost draconian requirements regarding time windows: within the critical 10-to-15-second interval between solder immersion and air-knife leveling, a temperature fluctuation of just five degrees can result in vastly different outcomes. Yet, this strict requirement has actually compelled factories to standardize their procedures; consequently, the defect rate on that specific production line is now even lower than it was when they were using lead-based solder.

Some people constantly complain that the coating produced by lead-free HASL is too thin and prone to oxidation. However, I have seen boards that underwent a “double-HASL” treatment—a process involving a second pass of hot air leveling—remain stable in terms of soldering performance even after sitting in storage for six months. The key lies in the precision of the manufacturing process, not in blindly chasing after coating thickness.

At times, I feel that the industry’s understanding of “environmental friendliness” is overly superficial. Switching to lead-free materials is merely the first step; the more practical and pressing challenges involve finding ways to reduce equipment energy consumption and minimize waste generation.

When it comes to PCB surface finishes, I personally place greater emphasis on the distinct performance characteristics and practical experiences offered by different options within specific application scenarios. I have often witnessed designs where, in a misguided pursuit of lower costs, manufacturers forced the use of an unsuitable surface finish—only to have the project “crash and burn,” resulting in a net loss rather than a gain. Take HASL PCBs, for instance: many people blindly opt for them simply because they are inexpensive, without considering their specific technical requirements. I once handled a project where the use of a standard HASL finish led to poor soldering quality on fine-pitch pins; the issue was ultimately resolved only after we switched to a different surface finish process.

Finally, regarding the issue of “tin whiskers” in the HASL process—it is actually not nearly as terrifying as it is often made out to be. Nowadays, many manufacturers incorporate special additives into their lead-free hot air solder leveling (HASL) processes to inhibit whisker growth; the key lies in finding a reliable supplier. I have worked with several manufacturers whose proprietary formulas indeed offer effective control over this issue, though it is essential to conduct on-site factory audits to verify this personally. Some smaller factories, in an effort to cut costs, may skimp on materials or workmanship, which often leads to problems down the line.

The storage environment has a far greater impact on PCBs than one might imagine. On one occasion, due to poor humidity control in our warehouse, an entire batch of OSP-treated boards oxidized before they could even be put into production, resulting in significant financial loss. Consequently, we implemented a strict policy requiring boards treated with different surface finishes to be stored separately, with temperature and humidity levels meticulously monitored and recorded—a crucial detail that many people tend to overlook.

Regarding surface flatness, one must evaluate the requirements in the context of the actual components being used. Not every application demands absolute flatness; while large-format BGAs, for instance, certainly require a high degree of flatness, standard resistors and capacitors are typically served perfectly well by a standard HASL finish. I have encountered instances where people insisted on using an Electroless Nickel Immersion Gold (ENIG) finish on simple double-sided boards—a choice that amounts to nothing more than a wasteful drain on the budget.

As for components with a 0.5mm pitch, most mainstream surface finishes available today are capable of handling them, provided that processing precision is strictly maintained. During a mass production run, we once discovered that a specific batch of boards suffered from slightly off-spec pad dimensions, resulting in a drop in yield; subsequent investigation revealed that the manufacturer had failed to perform adequate maintenance on their equipment. This underscores the critical importance of conducting periodic audits of a supplier’s production processes.

Different surface finishes exhibit significant variations in their tolerance for multiple reflow cycles. For complex assemblies that require passing through the reflow oven multiple times, it is highly advisable to conduct process validation in advance rather than waiting until mass production begins to uncover potential issues. Having learned this lesson the hard way ourselves, we subsequently incorporated a simulation and testing phase into our new product introduction process.

Ultimately, selecting a surface finish is akin to getting fitted for prescription glasses: it requires a tailored approach. There is no such thing as an inherently “good” or “bad” finish—only the question of whether or not it is suitable for the specific application. The key lies in understanding the genuine requirements of your product, rather than blindly following trends or single-mindedly chasing the lowest cost. Whenever you face this decision, ask yourself: In what environment will this board actually operate? What is its projected service life? What is the available budget? Once you have clearly defined these parameters, the correct answer will naturally emerge.

In closing, the industry is constantly evolving, and new surface finish technologies are continually emerging; therefore, maintaining a commitment to continuous learning is essential. However, the core principle remains unchanged: the *right* finish—the one best suited to the specific context—is always the *best* finish. There is no need to place excessive, blind faith in any single, specific technology.

I have observed that when it comes to selecting a surface finish for PCBs, many people tend to fall into a common trap: the assumption that a more expensive option is automatically a superior one. In reality, every surface finish technology has its own unique set of scenarios and applications for which it is best suited. I recall a project we worked on last year where we utilized the HASL (Hot Air Solder Leveling) process for a design featuring fine-pitch components, and the results were excellent. At the time, some members of the team insisted on using Electroless Nickel Immersion Gold (ENIG)—arguing it felt more “premium”—but during the testing phase, we discovered that soldering was actually less smooth than expected. We subsequently switched to lead-free HASL; not only did this cut costs by a third, but it also significantly reduced issues related to cold joints.

Nowadays, many engineers have a knee-jerk reaction to select ENIG the moment they see a 0.4mm pitch—but this is truly unnecessary. Unless you are dealing with special cases requiring long-term storage, such as military or aerospace applications, it is simply not required. For standard consumer electronics—which typically hit the market just a few months after production—HASL is perfectly adequate, especially given that modern lead-free processes offer significantly improved oxidation resistance.

The most extreme example I’ve encountered involved a client who absolutely insisted on applying an ENIG finish to the mainboard of a TV remote control, claiming it was to enhance the product’s “sense of quality.” In reality, that board didn’t contain a single fine-pitch component; they ended up spending twice the cost for absolutely no practical benefit.

When selecting a surface finish, you must first clearly consider the actual operating environment the product will face, rather than blindly following trends. If your board needs to withstand multiple high-temperature reflow cycles, then, by all means, you should consider a more robust finish. However, in the vast majority of cases, the adaptability of lead-free HASL is far greater than people realize.

Recent testing has even revealed that when paired with the HASL process, certain new types of flux actually have a *smaller* impact on high-frequency signals than some chemical plating finishes. This is a point particularly worth noting when designing RF (Radio Frequency) circuits.

Sometimes, the simplest solution proves to be the most reliable. After all, the primary mission of a circuit board is to function stably—not to compete over which surface finish looks the “coolest.”

I recently had a conversation with a friend who works in PCB manufacturing, and we observed an interesting phenomenon: many factories are still relying on outdated methods to treat board surfaces. They believe that sticking to processes that have been in use for decades is the safest and most reliable approach. However, the reality is that the landscape of the electronics industry has undergone a radical transformation.